A common type of integrated circuit device is a metal-oxide-semiconductor field effect transistor (MOSFET). A MOSFET is a field effect device that includes a source region, a drain region, a channel region extending between the source and drain regions, and a gate provided over the channel region. The gate includes a conductive gate structure disposed over and insulated from the channel region by a thin oxide layer.
Power MOSFET devices are widely used for high voltage circuit applications, e.g., greater than 200 volts. Examples of traditional MOSFET device structures for power applications include U.S. Pat. Nos. 5,869,875, 5,821,144, 5,760,440, and 4,748,936. Each of these devices has a source region and a drain region separated by an intermediate region. A gate structure is disposed over a thin oxide layer over the metal-oxide-semiconductor (MOS) channel of the device. In the on state, a voltage is applied to the gate to cause a conduction channel to form between the source and drain regions, thereby allowing current to flow through the device. In the off state, the voltage on the gate is sufficiently low such that no conduction channel is formed in the substrate, and thus no current flow occurs. In this condition, high voltage is supported between the drain and source regions.
Power transistors are often designed with interdigitated source and drain regions. Such a device structure is disclosed in U.S. Pat. No. 6,084,277, which is assigned to the assignee of the present application. The ""277 patent teaches a lateral power MOSFET or transistor having an improved gate design that provides a large safe operating area (SOA) performance level and high current capability with moderate gate speed to suppress switching noise.
Many power integrated circuits (ICs) contain one or more large high-voltage output transistors that control the current flow to one or more external loads. In a switch-mode power supply IC, for example, a single large high-voltage output transistor controls the current through the primary winding of a transformer and thereby controls the power delivered by the power supply. In certain applications it is also useful to include an additional high-voltage transistor on the same silicon substrate to provide a lower current coupled to, say, an external capacitor, for the purpose of assisting in the start-up of the chip or other external circuit. Such an additional high-voltage transistor is frequently referred to as an xe2x80x9cofflinexe2x80x9d transistor, even though it resides in the same substrate as the high-voltage output transistor. (In the context of the present application, the term xe2x80x9coffline transistorxe2x80x9d refers to a transistor having its drain region coupled to the same external line voltage as the output transistor, but which has its gate connected to a different internal circuit node than that of the output transistor.)
FIG. 1 shows a typical prior art power device 10 that includes an integrated circuit 11 housed within a chip carrier package. Integrated circuit 11 has an offline transistor 12 located in the upper right corner of the chip and an output transistor 13 located in another area of the same substrate. In conventional manner, bonding wires facilitate electrical connection between the bonding pads located on IC 11 and the various pins of the chip package. For example, FIG. 1 shows a bonding wire 21 connected between a bonding pad 20 located on the drain electrode 14 of offline transistor 12 and pin 19 of device 10. Similarly, wires 17 connect bonding pads 16 on drain electrode 15 of output transistor 13 to pins 18 and 19. Also shown are wires 28 connecting bonding pads 27 to pin 29, and wire 24 connecting pad 25 to pin 23. By way of example, bonding pads 27 and pin 29 may provide a connection to ground for IC 11, whereas pad 25 and pin 23 may connect to an external capacitor utilized for start-up purposes.
The prior art approach of FIG. 1 has several drawbacks. First, the offline transistor 12 is large and occupies a significant portion of silicon area. Offline transistor 12 also needs its own bonding pad 20, which is quite large relative to the active area of the offline transistor. This greatly reduces the area efficiency-of offline transistor 12. Secondly, the inductance of the drain bond wires 17 and 21 decouples offline transistor 12 from output transistor 13 during fast switching transients. This latter effect limits the ability of the output transistor to protect the offline transistor from potentially destructive high voltages that may be present at pins 18 and 19 of power device 10.
Therefore, what is needed is a power IC that overcomes the disadvantages inherent in the prior art.